Have you ever noticed a particular scent upon entering a room,
and then not noticed it ten minutes later? This is due to
olfactory fatigue. The olfactory sense is unique because it
relies on mass, not energy to trigger action potentials. Your
ears do not "stop" hearing a sound after a certain
period of time, nor do your eyes stop seeing something you may be
staring at. This is because both the ears and the eyes rely on
energy to trigger them, not mass. In the nose, once a molecule
has triggered a response, it must be disposed of and this takes
time. If a molecule comes along too quickly, there is no place
for it on the olfactory hairs, so it cannot be perceived. To
avoid olfactory fatigue, rabbits have flaps of skin that open and
close within the nostrils. This allows for short, quick sniffs
and lets the rabbit "keep in close odor contact with its
environment." When we wish to fully perceive a scent, we
humans also smell in quick, short sniffs, often moving the source
of the smell in front of one nostril then the other. This
behavior also prevents odor fatigue. (Stoddard
& Whitfield, 1984)